Traditionally a seat belt comprises a length of seat belt webbing connected at three points to load bearing parts of a vehicle. Typically one end is bolted to a door sill on one side of the seat, and is arranged to pass laterally across the hips of the vehicle occupant to a buckle mechanism fixed to the vehicle on the opposite side of the seat, and then diagonally across the torso of the vehicle occupant to a further fastening point on the B pillar of the door. The buckle mechanism engages a buckle tongue slideably attached to the webbing.
To increase the comfort of the vehicle occupant restrained by the seat belt a retractor is attached to the pillar end of the webbing. This allows the webbing to pay out under relatively low loads to enable limited movement of the vehicle occupant, for example to reach in-car entertainment controls or storage compartments. The retractor is biased to keep the webbing relatively taut about the vehicle occupant and a locking element is included to lock the retractor against webbing pay out in the event of a crash being detected. For example, an acceleration sensor activates if the vehicle undergoes rapid acceleration or deceleration indicative of a crash.
In recent years, pretensioners have been introduced to rapidly pull in a length of seat belt webbing to tighten the seat belt about the vehicle occupant in of a crash. This takes up any slack that may have developed in the seat belt and helps to more correctly position the vehicle occupant in the seat to maximize the effect of the seat belt protection and of any secondary safety restraint such as an airbag.
Pretensioners comprise a force reservoir such as a pyrotechnically operated gas generator to provide an impulse of sufficient magnitude to tighten the seat belt in a short space of time, ideally before the crash takes full effect. A typical known pretensioner may use rotational means to wind in a length of seat belt webbing, for example by rotating the retractor spool in a webbing rewind direction to take in the required length of webbing prior to the retractor locking against webbing pay out.
However, known pretensioners tend to be bulky, and are particularly difficult to use for the driver and front passenger seats of a two door vehicle because of the requirement to allow access to the rear of the vehicle past the front seats. Using a traditional retractor pretensioner mechanism in a front seat of a two door vehicle causes an unacceptable obstruction.
Seat travel is greater in a two door vehicle than in a four door vehicle to provide access to the rear seat and to accommodate this the door sill end of the webbing is usually attached to a so-called slider bar of well known design, instead of being bolted to the floor. This allows the sill end of the webbing to be moved longitudinally forward and rearward to facilitate rear seat access and front seat movement. The present invention provides an improved pretensioning mechanism that can be used in two-door, front seat applications.